1. Field of the Invention
The present invention relates to a color image reading device for entering color image data into an apparatus for electrically processing image information such as a digital copying machine, a facsimile or an electronic file.
2. Description of the Prior Art
As an example, in a digital color copying machine, a colored original to be copied is color-separated with color filters, and is photoelectrically read for example with a linear sensor having photoelectric converting elements such as photodiodes. The output signals of differenct colors from the linear sensor contain a dark current of the linear sensor. Consequently faithful reproduction of the color original image cannot be expected if a print is made from the output signals of the linear sensor.
Color signals required for image printing are generally prepared through a color conversion process on color information obtained by reading a color original with color separation.
The form of signals can be roughly classified as analog signals and digital signals. In comparison with an analog signal, a digital signal has certain advantages such as being less susceptible to the influence of noise, but involves certain specific drawbacks such as digitizing error missing bit. Thus, if the color information obtained by color separation of an original is processed and subjected to color conversion in a digital manner, a satisfactory color balance may not be maintained since the above-mentioned drawbacks of the digital signals may hinder exact color conversion.
Also the accuracy of the color signals may suffer a phase matching thereof conducted in the above-mentioned color converting process.
For the purpose of photoelectrically reading the density of a color original image, there is already known a linear sensor consisting of a linear array of plural photosensor elements. In the case of reading a black-and-white original image with a resolving power of 16 pixels/mm on the shorter side (ca. 210 mm) there will be required a linear sensor with ca. 3,500 photosensor elements. It is however extremely difficult to prepare so many photosensor elements, without defects and with a substantially uniform sensitivity, on the same substrate, so that such linear sensor is impractical in terms of cost, unless a significant improvement is made for example in the production yield. Also for reading a color image there are required tripled photosensor elements which further reduce the practicality of such sensor.
It has therefore been considered to arrange plural linear sensors along the scanning direction to read the image of a line using the plural linear sensors. Such structure, allowing reduction in the number of photosensor elements to be prepared on a substrate, improves the production yield and is capable of resolving the above-mentioned cost problem to a certain extent.
On the other hand, a color image reading provides image information about three times larger than a monochrome image reading, since the color image has to be separated into plural elementary colors, for example three colors. A high-speed processing of such color image information requires high-speed devices in the components constituting the processing circuit, thus leading to a high cost. Also the image reading speed is limited by the processing speed of such components.
Color image reading with a linear sensor can be achieved either by moving the optical image with respect to a fixed linear sensor or by moving the linear sensor with respect to a fixed original image, but the latter is preferred in consideration of the miniaturization of the apparatus or ease of control.
In such case, in order to transmit the color signals from the moving linear sensor to a processing unit in the apparatus, there is required a signal line of a length at least equal to the moving distance of the sensor. However such apparatus is usually provided with various noise-generating components such as a sensor-driving motor or a high-voltage transformer which may affect and disturb the color signals transmitted in said signal line. Thus exact color conversion cannot be expected and satisfactory copying of a color image cannot be achieved.